The present invention relates to a nondestructive method for determining at least one physical property of a wood member. It further relates to a method of optimizing value of the wood member during further processing
It has been long known to use nondestructive testing methods for determining some physical property of a wood member which relates to its strength or soundness. Items such as logs, utility poles, or lumber intended for engineering applications are routinely tested. One means of doing this is to induce a stress wave within the material and note a response characteristics; e.g., the time of travel of the wave, to infer the property being studied. The stress wave may be induced by striking the material with a hammer and noting the response by means of an accelerometer in contact with the piece. Another way is to direct a sonic pulse at the material, either by a transducer in direct contact, or by an external transducer through an air gap. The sonic pulse may be swept through a range of frequencies since the impedance of the wood is high to any but frequencies at or very near the resonance point, or to harmonics of this frequency. Unfortunately, the energy delivered by the sonic pulse is relatively low and the resulting stress wave in the log may be difficult to detect and process.
A number of earlier investigators have looked at varying means of using sonic pulses to determine physical properties of wood members. An example is found in published PCT Application WO 02/08747 to Harris. Unfortunately, in systems using sonic pulses the sound may be very loud. At best the noise can be an annoyance. It may require those working nearby to wear ear protection in order to prevent hearing damage.
Systems using mechanically induced shock waves that measure end-to-end transit time of the wave in the sample have been in use for evaluating logs and assigning them for optimum use based on the determined elastic modulus. Such a system is described in Snyder et al, U.S. Pat. No. 6,026,689. The system is normally employed on a log ladder in a sawmill or merchandiser. With this method the logs must be even ended for access to the pneumatic hammer. It is also necessary for the log to remain stationary for the short time required for the test. The need for the logs to be even ended poses some difficulty since the heavy logs, which are frequently of varying lengths, must be brute force adjusted into the proper position
All of the systems noted above suffer some deficiency when used in an industrial environment such as a sawmill or log sort yard. These environments have inherently high background noise. This greatly complicates the use of non-contact systems and makes detection of the weak stress wave induced in the log extremely difficult to separate from the noise. Even-ending of the logs for access to a pneumatic hammer poses a considerable and sometimes insolvable problem Further, the logs are often moving at a high rate of speed and the time window in which a reading may be made is frequently considerably less than a second. The present invention is an improvement in the known systems and successfully overcomes many of the problems just noted.
The present invention is a non-contact method for determination of one or more physical properties in a wood member such as a log or structural timber. When the term xe2x80x9clogxe2x80x9d is used, it is a term of convenience and should be read with sufficient breadth to include any elongated wood member being tested for structural properties.
The method employs projecting a small amount of a liquid or solid against the end of the log with sufficient energy to initiate a stress wave. By small amount is meant generally less than about 50 g. The term xe2x80x9cprojectingxe2x80x9d implies travel through space for a distance in the range of about 0.3-3 m (1-10 ft). The time of travel of the stress wave to the end of the log and back may be measured by an accelerometer or other suitable transducer in contact with the log. Alternatively, a non-contact transducer such as a laser Doppler vibrometer may be used to receive the returned signal. The returned signal is converted into the frequency domain and the resonant frequency of the log is determined.
The speed of the stress wave transmitted into the log can be readily determined by the equation S=2Lf, where L is the length of the log and f is the resonant fundamental frequency. Stress wave speed is known to relate directly to modulus of elasticity (MOE), with lower speeds indicating a lower MOE. Knowledge of the MOE can then be used to determine subsequent use of the log. Low modulus logs can be sawn into dimension lumber sizes or grades where bending strength is not critical or can even be directed for production of pulp chips or composite panels. U.S. Pat. No. 6,026,689 is descriptive of how knowledge of MOE can be used to maximize product value of saw logs.
A high pressure pulse of water is an example of a benign liquid that may be projected against the end of the log to initiate the stress wave. By high pressure is meant at least about 3450 kPa (500 psi), more preferably at least about 8300 kPa (1200 psi). Other liquids are suitable but water is preferred. While various equipment arrangements are possible to create the high pressure water pulse, they will in general have the common elements of a high pressure pump, an accumulator to hold the requisite amount of high pressure air, a container to store the amount of water needed, a valve to release the air from the accumulator and project the water, and a nozzle or similar orifice to direct the water at the target. A small ice pellet is a preferred example of a solid material that may be directed at the end of the log to initiate the stress wave. Other solid materials may be used as pellets but ice is much preferred. The ice pellet will fragment when it hits the log and generally adhere for a short period of time. There is no need for recovery of the pellets, as with other solid materials, and the ice fragments readily dissipate into the use environment.
It is an object of the present invention to provide an improved method for evaluation of at least one physical property of a wood member.
It is a further object to provide a method using a high pressure liquid pulse to excite resonance within such a member.
It is also an object to direct a small ice pellet at the end of the member to initiate the stress wave.
It is another object to provide a method for non-destructive evaluation of a wood member on a log ladder that does not require the logs to be even ended for testing.
It is one more object to provide a fully non-contact method for determination at least one log physical property.
These and many other objects will become readily apparent upon reading the following detailed description taken in conjunction with the drawings.